1. Fluidized Catalytic Cracking
Catalytic cracking of heavy hydrocarbon fractions is one of the major refinery operations for converting crude oils into desirable fuel products such as gasoline. In a fluidized catalytic cracking (FCC) process, a heavy hydrocarbon feed is contacted with fluidized catalyst particles in a cracking, or reaction, zone at a temperature of about 800.degree. to 1100.degree. F. The heavy hydrocarbons crack under these conditions, but unfortunately, the conditions also cause the deposition of carbonaceous coke on the catalyst particles. The resulting cracked hydrocarbon products are then separated from the coked catalyst and withdrawn from the cracking zone. The coked catalyst is, in turn, stripped of volatiles and passed to a catalyst regeneration zone.
In the catalyst regenerator, the coked catalyst is contacted with a gas containing a controlled amount of molecular oxygen to burn off a desired portion of coke from the catalyst and to simultaneously heat the catalyst to the high temperature desired when the catalyst is again contacted with the hydrocarbon stream in the cracking zone. The catalyst is then returned to the cracking zone, where it vaporizes the hydrocarbons and catalyzes hydrocarbon cracking. The flue gas is separately removed from the regenerator. This flue gas, which may be treated to remove particulates and carbon monoxide, is normally passed into the atmosphere.
2. Cracking Catalysts
Although neither silica nor alumina alone possesses catalytic activity for cracking reactions, it is known that the incorporation of a minor amount of alumina into silica results in a material having such catalytic activity. According to Gates, B. C. et al. Chemistry of Catalytic Processes (McGraw-Hill 1979), the Al-O-Si bonds which are formed contain acid sites which are responsible for the catalytic activity. The maximum catalytic activity is obtained when the alumina is present in a concentration of about 25 weight percent. Higher alumina concentrations show decreased activity because the formation of Al-O-Al bonds decreases the acidity of the material. By 1960, the most widely used cracking catalysts in commercial FCC units were amorphous mixtures of silica and alumina containing about 10 to 25 weight percent alumina.
About 1960 it was discovered that certain crystalline aluminosilicates, also called zeolites, have catalytic activities many times that of their amorphous counterparts. Since these zeolites had previously been used as adsorbents, they were also widely known as molecular sieves. The zeolites are composed of oxygen-cornered silica and alumina tetrahedra joined together in structures which contain uniform pores of exceedingly small size, the cross-sectional diameter of the pores being in the range of about 5 to 20 angstroms, and often being in the range of about 6 to 12 angstroms. For example, it is known that the building blocks of the Y-type zeolite described in U.S. Pat. No. 3,130,007 is a cage-type unit cell which contains 192 silica and alumina tetrahedra and which has pore diameters of about 7 angstroms.
Zeolites such as Zeolite Y have such increased activity that catalyst particles composed entirely of the zeolite overcrack the hydrocarbon feedstocks when used in commercial FCC units. Therefore, it became the practice to use catalyst particles containing a minor amount of a zeolite dispersed in a major amount of an amorphous silica-alumina of the type which previously had been used as a cracking catalyst. The past 20 years have seen tremendous advances in the use of zeolites as cracking catalysts. In addition to the 34 known natural zeolites, about 100 zeolites have been synthesized which have no natural counterpart. Furthermore, it has recently been discovered that crystalline borosilicates can be synthesized which possess activities equal to, or greater than, the crystalline aluminosilicates.
The above-mentioned zeolites generally occur naturally or are prepared in the sodium form in which sodium cations are positioned at various sites in the crystalline structure of the sieve. The sodium cations balance the negative charges resulting from the substitution of the trivalent boron or aluminum atoms for the tetravalent silicon atom in the tetrahedral structure. The amount of sodium cations originally in the sieve is thus a function of the ratio of boron and/or aluminum atoms to silicon atoms and can range from about 2.0 to 15.0 weight percent. For example, a Y-type zeolite contains about 57 sodium cations per unit cell and this represents about 10.0 weight percent of the zeolite. The 57 cations are distributed among the 112 available cation sites (16 Type I, 32 Type I', 32 Type II, and 32 Type II').
It is now known that a zeolite possessing increased activity and increased thermal stability can be formed by exchanging the sodium cations for hydrogen ions or rare earth metal ions. In the ion exchange process, the sodium form zeolite is generally dispersed at elevated temperatures in an aqueous salt solution containing an excess of the cation to be exchanged. After a suitable period of time, the zeolite is removed from the solution, washed with deionized water, and then dried. The ion exchange treatment generally exchanges about 50 to 90 percent of the sodium ions. For example, in a Y-type zeolite all but 16 sodium ions per unit cell are relatively easily exchanged out. Thus, about 72 percent (57-16/57) of the sodium cations are generally exchanged in a single ion exchange treatment. It is believed that these 16 sodium cations are located in the Type I sites which are relatively inaccessible.
Various processes have been taught for producing zeolites with low levels of sodium. For example, in Gates, B. C. et al., Chemistry of Catalytic Processes (McGraw-Hill 1979), it is taught that essentially complete removal of sodium cations from an X- or Y-type zeolite is obtained by exchanging the zeolite, then calcining to 350.degree. C., and then exchanging again. Gates et al. teach that the calcination step replaces the 16 sodium cations in the Type I sites with the cations that have been exchanged into the structure and that the remaining sodium cations, now in more easily accessible sites, are exchanged out in the post-calcination exchange.
Alafandi, U.S. Pat. No. 4,192,778, teaches a process in which over 90 percent of the sodium cations in a faujasite zeolite are exchanged by rare earth cations. The process comprises forming a water slurry of rare earth salts and a zeolite of the faujasite type, and then heating at superatmospheric pressure at a temperature of about 250.degree. to 500.degree. F. Alafandi further teaches that the resulting zeolite has improved hydrothermal stability and catalytic activity. Other examples of teachings relating to low sodium zeolites include Maher, U.S. Pat. No. 3,402,996 (first exchange-calcination-second exchange) and Sherry, U.S. Pat. No. 3,677,698 (exchange at superatmospheric pressure).
3. Sulfur Oxide Emissions
The hydrocarbon feeds processed in commercial FCC units normally contain sulfur chemically combined in the hydrocarbon molecules. When the hydrocarbon feed is processed in the cracking zone, about 90 percent or more of the sulfur is converted either to normally gaseous sulfur compounds such as hydrogen sulfide and carbon oxysulfide, or to normally liquid organic sulfur compounds. These sulfur compounds are carried along with the vapor products recovered from the cracking reactor. Generally about 40 to 60 percent of this sulfur is in the form of hydrogen sulfide.
Provisions are conventionally made to recover hydrogen sulfide from the effluent of the cracking reactor. Typically, a low-molecular-weight gas stream is separated from the C.sub.3 -plus liquid hydrocarbons in a gas recovery unit, and the gas is treated by scrubbing with an amine solution to remove the hydrogen sulfide. The hydrogen sulfide is often converted to elemental sulfur by any of the conventional techniques known to the art, such as the Claus process.
Unfortunately, the other 10 percent or less of the sulfur in the feed is invariably transferred to the catalyst particles as part of the coke formed during the cracking reaction. This sulfur is eventually cycled from the cracking zone along with the coked catalyst into the regenerator. In the regenerator, the sulfur is burned and gaseous sulfur oxides, sulfur dioxide and sulfur trioxide, are formed. Conventional flue gas treatments for removal of particulates and carbon monoxide do not remove the sulfur oxides. As a result, the flue gas from an FCC regenerator which is vented to the atmosphere often contains 1200 parts per million by volume (ppmv) or more of sulfur oxides.
Sulfur oxides are a serious air pollutant since they can react with water in the atmosphere to form sulfuric acid. Therefore, the oil refining industry has been searching for a suitable means of reducing sulfur oxide emissions. Furthermore, the Environmental Protection Agency, acting under the Clean Air Act, is considering the proposal of regulations which would limit sulfur oxide emissions from FCC regenerators to an amount in the range of about 100 to 400 ppmv.
4. Absorbents to Control Sulfur Oxide Emissions
One promising approach to reducing sulfur oxide emissions from an FCC regenerator is to add a metallic reactant to the circulating cracking catalyst which absorbs the sulfur oxides produced in the regenerator. The absorbed sulfur is then liberated as a gas comprising hydrogen sulfide in the cracking zone. This approach is so attractive because the sulfur thus shifted from the regenerator flue gas to the reactor effluent is simply a small addition to the large amount of hydrogen sulfide invariably present in the reactor effluent. The small added expense, if any, of removing even as much as 5 to 15 percent more hydrogen sulfide from an FCC reactor gas stream by available means is substantially less than, for example, the expense of separate feed desulfurization or flue gas scrubbing to reduce the level of sulfur oxides in the regenerator flue gas.
In more detail, it is believed that a chemical reaction occurs between the metallic reactant/absorbent and the sulfur oxides which results in the formation of nonvolatile inorganic sulfur compounds, such as sulfites and sulfates. This chemical reaction is reversible and can be summarized in a simplified manner by the following equations: EQU M.sub.x O+SO.sub.2 .fwdarw.M.sub.x SO.sub.3 EQU M.sub.x O+SO.sub.3 .fwdarw.M.sub.x SO.sub.4 EQU MxO+SO.sub.2 +1/2O.sub.2 .fwdarw.M.sub.x SO.sub.4
where M is the metal and x is the ratio of the oxidation state of the oxide ion to the oxidation state of the metal. At very high temperatures, these sulfites and sulfates can undergo partial decomposition to liberate the original sulfur oxides and absorbent. Therefore, the absorption of sulfur oxides is preferably conducted at temperatures below about 1600.degree. F.
It is further believed that the combination of a hydrocarbon feed and a cracking catalyst in the cracking zone provides a reducing environment which effects a conversion of absorbed sulfur oxides to hydrogen sulfide while simultaneously reactivating the absorbent for further absorption of sulfur oxides. The removal of absorbed sulfur oxides can be summarized in a simplified manner by the following equations: EQU M.sub.x SO.sub.3 +3H.sub.2 .fwdarw.M.sub.x O+H.sub.2 S+H.sub.2 O EQU M.sub.x SO.sub.4 +4H.sub.2 .fwdarw.M.sub.x O+H.sub.2 S+3H.sub.2 O EQU M.sub.x SO.sub.3 +3H.sub.2 .fwdarw.M.sub.x S+3H.sub.2 O.fwdarw.M.sub.x O+H.sub.2 S+2H.sub.2 O EQU M.sub.x SO.sub.4 +4H.sub.2 .fwdarw.M.sub.x S+4H.sub.2 O.fwdarw.M.sub.x O+H.sub.2 S+3H.sub.2 O
where M and x are as above. The removal of absorbed sulfur oxides from the absorbent is generally improved by contacting the absorbent with added steam. It is believed that at least some metal sulfide is formed according to the latter two above equations and that added steam serves to promote the conversion of these metal sulfides to hydrogen sulfide with simultaneous reactivation of the absorbent.
A number of different ways have been suggested for adding the metallic reactant/absorbent to the circulating cracking catalyst, for example: (1) as a separate particulate; (2) as part of the catalyst matrix; (3) deposited upon the surface of the catalyst; and (4) ion exchanged into the zeolite.
5. Rare Earth Metals as Absorbents
As previously seen, rare earth metals have been widely used to exchange out sodium cations in zeolites and to thus give improved activity and thermal stability. The rare-earth-form zeolites are conventionally dispersed in silica-alumina matrices for use as cracking catalysts. Rare earth metals and their compounds have also been taught to be suitable sulfur oxide absorbents.
Longo, U.S. Pat. No. 4,001,375, discloses a process for removal of sulfur oxides from a gas which involves absorbing the sulfur oxides with cerium oxide followed by regeneration of the spent cerium oxide by reaction with hydrogen gas. This regeneration step results in the formation of a gas which contains a 1:1 ratio of hydrogen sulfide to sulfur dioxide and which can be fed directly to a Claus-type sulfur recovery unit for conversion into elemental sulfur. It is further disclosed that the cerium oxide may be supported on an inert support such as alumina, silica, or magnesia.
Vasalos, U.S. Patent 4,153,534, discloses a process for the removal of sulfur oxides from an FCC regenerator flue gas through the use of a zeolite-type cracking catalyst in combination with a regenerable metallic reactant. Suitable metallic reactants comprise one or more members selected from the group consisting of sodium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, the rare earth metals and lead, in free or combined form.
Vasalos teaches that the metallic reactant can be present in many forms. First of all, it can be present as a powder which is separate from any support or it can be incorporated onto a suitable support. Suitable supports include zeolite-type cracking catalysts, amorphous cracking catalysts, and substantially inert substances. If the metallic reactant is incorporated onto a support, the incorporation can be performed by: (1) ion exchange, (2) impregnation, (3) adsorption, or (4) some other means. Vasalos states that impregnation and adsorption can be performed with the support either before it is introduced into the cracking process cycle or afterwards by introducing the metallic reactant into the cracking process cycle and thereby incorporating it in situ onto the support.
At col. 14, lines 63 et seq., Vasalos states that "The key features of activity and stability are more easily attainable by introducing the metallic reactant into the cracking process cycle and incorporating it into the solid particles in situ, rather than compositing it with the cracking catalyst during manufacture of the cracking catalyst. Introducing the metallic reactant into the cracking process cycle and incorporating it in situ as opposed to compositing it with the cracking catalyst during cracking catalyst preparation has been found to result in greater reduction in emissions of sulfur oxides in regeneration zone flue gases." None of the operating examples show the use of a metallic reactant incorporated onto a support solely by ion exchange.
In discussing suitable zeolite-type cracking catalysts, Vasalos notes that the zeolites are usually made in the sodium form. Then the sodium component is usually reduced to as small an amount as possible, generally less than about 0.30 weight percent, through ion exchange with hydrogen ions, hydrogen-precursors such as ammonium ions, or polyvalent metal ions, including calcium, strontium, barium, and the rare earths, such as cerium, lanthanum, neodymium, and naturally-occurring rare earths and their mixtures.
6. Free Alumina as Absorbents
Alumina chemically combined with silica has been widely used in FCC cracking catalysts in both the amorphous and crystalline forms. Alumina which is not chemically combined, often called "free alumina," has not been widely used since it has low catalytic activity. However, it is known that at least some forms of free alumina can be used as the metallic reactant/absorbent for the removal of sulfur oxides from regenerator flue gas.
An article entitled "Selection of Metal Oxides for Removing SO.sub.2 from Flue Gas" by Lowell et al. in Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 3, 1971, is addressed to a theoretical evaluation of the possible use of various metal oxides to absorb sulfur dioxide from a flue gas. The authors evaluated 47 metal oxides from which they selected a group of 16 potentially useful single oxide absorbents. One of the 16 is aluminum oxide. The evaluation was based on the assumption that the absorbents would be regenerated thermally and did not consider the possibility of regeneration under reducing conditions.
Blanton, U.S. Pat. No. 4,071,436, teaches that sulfur oxides can be removed from a regenerator flue gas by reaction with a "reactive alumina" component of a particulate solid introduced in the regenerator. The reactive alumina is preferably part of a solid particulate employed in addition to the conventional cracking catalyst. Blanton defines "reactive alumina" to be the weight fraction of alumina contained in a solid particle which reacts to form a sulfate of aluminum when the solid particle is treated in a specified manner. Blanton teaches that alumina which is chemically combined with silica, as in silicaalumina cogels and zeolites, normally contains no reactive alumina. Blanton further teaches that substantially pure alumina contains about 1 to 2 weight percent reactive alumina.
7. Physically Incorporated Rare Earth Metal Plus Free Alumina as Absorbents
Bertolacini, U.S. Patent Application Ser. No. 29,264, filed Apr. 11, 1979, discloses a process for removing sulfur oxides from a gas with an absorbent which comprises at least one inorganic oxide selected from the group consisting of the oxides of aluminum, magnesium, zinc, titanium, and calcium in association with at least one free or combined rare earth metal selected from the group consisting of lanthanum, cerium, praseodymium, samarium, and dysprosium. Bertolacini teaches that the rare earth metal(s) and the inorganic oxide(s) act together in a synergistic manner to afford a more efficient absorption of sulfur oxides from a gas than is possible if they are used separately.
Bertolacini further teaches that the preferred inorganic oxide is alumina and that, while any form of alumina is suitable, the gamma-alumina and eta-alumina forms are preferred because of their large surface areas. It is further taught that the rare earth metal is preferably physically incorporated with the inorganic oxide. At page 19, lines 20 et seq., it is taught that "the rare earth metal or metals, which are associated with one or more suitable inorganic oxides, are preferably used in a form which does not involve chemical incorporation within a zeolite. Consequently, the rare earth metal or metals of this invention for use in the absorption of sulfur oxides are preferably not incorporated into a zeolite, for example by ionexchange techniques, and are in addition to any such rare earth metal or metals which may be so incorporated in a zeolite. Such ion-exchanged rare earth metal or metals are not detrimental to the practice of this invention, but this form of rare earth metal is relatively inactive with respect to the absorption of sulfur oxides."
Brown, British Pat. No. 2,032,947, discloses a similar process in which sulfur oxides are removed with discrete particles of alumina having at least one rare earth compound supported thereon. The alumina particles may be a component of particles of a composite cracking catalyst or separate fluidizable entities other than cracking catalyst and physically admixed with the catalyst particles. At least a portion of the discrete alumina used as a support for the rare earth must be "free" alumina and in an "active" form.
As did Bertolacini, Brown specifically teaches that the rare earth metal is preferably physically incorporated with the inorganic oxide. At page 2, lines 78 et seq., Brown states that "In practice of the invention, the rare earth compound(s) must be supported on the alumina per se although one or more rare earth materials may be present with one or more constituents of the solid entities, of which the discrete alumina may be a component. For example, many present-day commercial composite zeolitic cracking catalyst contain rare earth such as cerium or a rare earth mixture associated with the zeolite component as a result of ion-exchange with cations originally associated with the zeolite. When such composite catalysts also contain discrete free alumina as a matrix component, the rare earth will normally not be supported or deposited to an appreciable extent on the discrete alumina in the matrix when conventional ion-exchange techniques are practiced to prepare the catalyst particles. For the most part, the rare earth will be present with the zeolitic component and, in this state or condition, the rare earth will not synergistically act with the alumina in the same catalyst particles to reduce sulfur oxide emissions as it will when the rare earth is supported on the alumina particles. Therefore, unless exchange with rare earth is carried out under conditions such that additional rare earth is deposited on at least a portion of the discrete alumina particles in the matrix of composite catalyst particles, it will be necessary to deposit rare earth on discrete alumina components on such catalyst particles by additional processing. This may be accomplished, for example, by impregnating the finished catalyst particles with rare earth or by impregnating the alumina component prior to incorporation of the alumina particles into the catalyst matrix."
8. Deactivation of Absorbents by Silica Migration
One major problem associated with the control of FCC regenerator sulfur oxide emissions by the use of an absorbent which circulates with the cracking catalyst is that the absorbent deactivates relatively quickly. It is believed that the deactivation occurs when amorphous silica, which is generally present in the catalyst matrix, migrates to combine with the absorbent. It was originally thought that the silica migration occurred only within a given particulate. However, Blanton, U.S. Pat. No. 4,259,176, teaches that amorphous silica can migrate from particles of high silica concentration to particles of lower silica concentration during circulation of the particles in an FCC unit. Accordingly, Blanton teaches a process using a zeolitic-type cracking catalyst containing little or no silica in its matrix.